The Tentacled Snake has evolved a unique method of catching prey.
The Tentacled Snake, Erpeton tentaculatum, is native to the jungle rivers and lakes of Vietnam, Thailand and Cambodia in southest Asia. It is the only member of its genus, but belongs to the Homalopsid family, which includes about fifty species in 28 genera, known as the “Asian Water Snakes” or “Mud Snakes”, which range from India across southeast Asia, the Philippines, Indonesia, and down to northern Australia.
The Tentacled Snake is smallish in size, averaging between two and three feet. There are two distinct color morphs, with some youngsters bearing dark splotches and others sporting dark stripes that run lengthwise along the body. The snake is entirely aquatic and inhabits stagnant river stretches and ponds. It is also commonly encountered in rice paddies. The young snakes are born live, eliminating any need for the females to leave the water to lay eggs.
The diet consists exclusively of fish, and the snakes have evolved an odd method of catching them. Although Tentacled Snakes have good vision, the water they inhabit is often muddy and murky, making it difficult to pursue prey underwater. So the snakes have developed a complex “ambush” behavior which has only recently become understood.
The snakes wrap their tails around an underwater plant and stretch their bodies out, unmoving, thereby mimicking the appearance of a stem or stick. To help them detect fish as they swim by in the murky water, Tentacled Snakes have evolved two protruding antenna-like projections that extend from their upper lip. When the fish is in range, the snake strikes with an extremely rapid motion. Tentacled Snakes are “rear-fanged”: they have enlarged grooved teeth at the back of their jaws which dribble a mild paralytic venom into their prey.
For many years, the snake was little-studied, and it was simply assumed that the tentacles performed some sensory role to allow fish to be identified and struck. But in 2011 biologist Kenneth Catania examined the Tentacled Snake more closely, and discovered that the process is much more interesting.
Although it was always assumed that the tentacles had some fish-catching function, it was never known exactly how they worked. Some authorities assumed they acted like lures to attract fish into striking range. Others speculated that they might be some sort of electric-field sensors. But when Catania examined the tentacle structures under an electron microscope, he found a dense network of nerve endings deep under the skin. These acted as extremely sensitive pressure sensors, which detected vanishingly minute changes in the position of the tentacles and allowed the snake to feel the tiny vibrations in the water caused by an approaching fish, then triangulate the prey’s position and speed even if it was unable to see it.
To hunt, the Tentacled Snake assumes a characteristic pose that has become known as the “J-posture”. While holding on to a plant stem or stick with its tail, the snake stretches out its body like a stiff rod—but it curls its head and neck inwards to form a J shape with a sharp curve. This forms the striking area: the snake only attacks when the prey fish is located at the proper spot inside this concave curve where it is within range.
While observing the snake’s hunting behavior, however, Catania noticed something odd: in every case, the fish prey would turn towards the snake’s strike, making it an easy catch. The Tentacled Snake seldom missed.
Using high-speed cameras, it became apparent that the snake was taking advantage of the fish’s own natural defensive reaction. When fish sense danger, they instinctively tense their body into a distinctive C shape, allowing them to then instantly flap their tail and swim away to escape. Because this “C-start” response is a reflex action and does not need to be consciously carried out by the fish, it is extremely rapid (it happens in around 30 milliseconds—literally faster than the blink of an eye). But the Tentacled Snake, with its own lightning-fast strike, is able to seize the fish before it can dart away.
Instead of striking directly at the fish’s head, however, the photography also showed that the Tentacled Snake aims for the spot where the fish’s head will be at the end of its C-start response. Apparently the snake knows that the fish’s reflex action always turns the fish so the ensuing tail flap will push it away from the danger, and so by aiming at the place where the fish’s head will be, the Tentacled Snake is able to seize the fish before it can initiate the escape motion. The strike happens so rapidly that the snake has to withdraw its eyes partially into their sockets to protect them from the water pressure, and therefore it is unable to see the fish during the motion. The snake is still capable of striking the predicted spot even if it experimentally blindfolded.
But that presents another puzzle: once the fish enters the curved “U” between the snake’s head and body, it might be able to sense either part of the snake, and therefore the automatic C-start reflex could turn the fish in either direction as it tries to escape. So how does the snake successfully predict which direction the fish will turn, allowing it to calculate its future position and make a successful strike?
Using sensitive hydrophones, Catania discovered that the snake itself was able to force the fish to turn in the direction needed for a successful catch. The instant the fish entered the most favorable spot for a strike, the Tentacled Snake would very briefly vibrate the portion of its body at the opposite side of the approaching fish. This vibration in turn immediately activates the fish’s escape reflex, which initiates an automatic dash away from the source of the disturbance—directly towards the snake’s head. The whole process takes less than 30 or 40 milliseconds.
The Tentacled Snake is therefore able to take advantage of the fish’s own innate defense mechanism, provoking the fish to make itself easier to catch and thereby become an easy meal. And since this process is entirely instinctive and requires no learned ability on the part of the snake, even newborn Tentacled Snakes that have ever seen a fish before are able to feint them into position and successfully strike at them before they can flee.